1. Field of the Invention
The present invention relates to a color synchronization method and a color laser printer which achieves color synchronization by providing a timing display on a timing belt to rotate a photosensitive drum.
2. Description of the Related Art
In general, black-and-white laser printers transfer only black ink onto a sheet of paper, and thus one organic photoconductive cell (OPC) photosensitive drum is used in one laser scanning unit (LSU).
To the contrary, color laser printers should transfer ink having four colors, such as black (K), magenta (M), yellow (Y), and cyan (C), onto the paper, and thus four LSUs and four OPCs photosensitive drums which correspond to the respective four colors are required. As shown in FIG. 1, a conventional color laser printer includes a black OPC photosensitive drum 100-K, a cyan OPC photosensitive drum 100-C, a magenta OPC photosensitive drum 100-M, a yellow OPC photosensitive drum 100-Y, LSUs 102-K, 102-C, 102-M, and 102-Y which radiate light onto the black, cyan, magenta, and yellow OPC photosensitive drums 100-K, 100-C, 100-M, and 100-Y, respectively, charged to a predetermined potential so as to form each electrostatic latent image, developing units 105-K, 105-C, 105-M, and 105-Y which develop the electrostatic latent image using a developer having four colors, a transfer belt 108 which receives the developed image from the black, cyan, magenta, and yellow OPC photosensitive drums 100-K, 100-C, 100-M, and 100-Y to form a color image, a transfer unit 110 which transfers the color image of four colors from the transfer belt 108 onto a sheet of paper P, and a fusing unit 115 which heats and squeezes the paper P and fuses the transferred image on the paper P. Reference numeral 104 denotes a toner supply container supplying toner to the respective developing units 105-K, 105-C, 105-M, and 105-Y.
As described above, in order to realize a color image, the conventional color laser printer includes the black, cyan, magenta, and yellow OPC photosensitive drums 100-K, 100-C, 100-M, and 100-Y and the four LSUs 102-K, 102-C, 102-M, and 102-Y which correspond to the four colors.
Each LSU 102-K, 102-C, 102-M, or 102-Y radiates light onto a photosensitive medium, such as the corresponding OPC photosensitive drum 100-K, 100-C, 100-M, or 100-Y, and forms the electrostatic latent image on the corresponding one of the black, cyan, magenta, and yellow OPC photosensitive drums 100-K, 100-C, 100-M, and 100-Y. In this way, when the corresponding image developed on each photosensitive drum is transferred onto the transfer belt 108, black, cyan, magenta, and yellow images sequentially overlap on an identical line of the transfer belt 108 to form the color image onto the transfer belt 108. In this case, only if a line transferred for each color is placed correctly in an identical position of the transfer belt 108, a quality of the color image is improved. Thus, devices for transferring the corresponding image for each color to a precise position of the transfer belt 108 have been developed. Likewise, transferring the corresponding image for each color in the identical position of the transfer belt 108 is referred to as color synchronization.
In the conventional color laser printer, in order to realize the color synchronization, as shown in FIG. 1, first through fourth holes 117a, 117b, 117c, and 117d, which correspond to each color, are bored at predetermined intervals on the transfer belt 108, a light source 118, such as an LED, is placed inside the transfer belt 108, and a light receiving portion 119 is provided opposite to the light source 118 such that the transfer belt 108 is placed therebetween.
As the transfer belt 108 rotates at a predetermined velocity, the first through fourth holes 117a, 117b, 117c, and 117d sequentially pass through a space between the light source 118 and the light receiving portion 119. When the first hole 117a passes through a place in which the light source 118 is installed, light radiated from the light source 118 passes through the first hole 117a and is received by the light receiving portion 119 disposed opposite to the light source 118 with respect to the transfer belt 108. The yellow image exposed and developed on the yellow OPC photosensitive drum 100-Y is transferred onto a predetermined line of the transfer belt 108 in response to a first received signal of the light receiving portion 119. Next, when the second hole 117b passes through the space between the light source 118 and the light receiving portion 119, light radiated from the light source 118 passes through the second hole 117b and is received by the light receiving portion 119. The magenta image exposed and developed on the magenta OPC photosensitive drum 100-M is transferred to the predetermined line of the transfer belt 108 in response to a second received signal of the light receiving portion 119.
In the same manner, when the third and fourth holes 117c and 117d pass through the space between the light source 118 and the light receiving portion 119, the cyan and black images exposed and developed on the cyan OPC photosensitive drum 100-C and the black OPC photosensitive drum 100-K are sequentially transferred onto the transfer belt 108 in response to third and fourth receiving signals received by the light receiving portion 119, respectively. Here, the yellow, magenta, cyan, and black images are sequentially transferred from the photosensitive drum for each color and should be repeatedly transferred to a predetermined precise location of the transfer belt 108. Likewise, in order to realize synchronization for each color, the positions of the first through fourth holes 117a, 117b, 117c, and 117d should be consistent with the positions of the photosensitive drums 100-Y, 100-M, 100-C, and 100-K, and a constant rotation velocity or a constant rotation period of the photosensitive drums 100-Y, 100-M, 100-C, and 100-K and the transfer belt 108 should be maintained.
However, in order to realize color synchronization using the above method, an additional light source for synchronization is required around the transfer belt, and thus a power consumption increases, and material and assembly costs increase. In addition, the photosensitive drums and the transfer belt rotate using different driving sources, and thus it is difficult that a rotation velocity of the photosensitive drums and the transfer belt are correctly consistent with each other. Thus, there is a limitation in sensing the positions of the holes formed on the transfer belt and in precisely adjusting a transfer position of the photosensitive drum. Furthermore, as the transfer belt is deformed or expanded due to wear and daily use, the relative positions of the first through fourth holes formed on the transfer belt 108 can be changed. As a result, a transfer time for each color may be detected incorrectly, and transfer positions for each color are inconsistent with one another, thereby a lower quality image is obtained.